On May 14,
1993, the New Mexico Office of the Medical Investigator was notified of the
unexplained deaths of a couple living in the same household in rural New Mexico:
a 21-year-old
woman and a 19-year-old man. Both died of acute respiratory failure — the man
within five days of the woman. By May 17, Indian Health Service physicians had
reported five deaths
from adult respiratory distress syndrome among previously healthy adults.
Surveillance was initiated for an influenza-like illness followed by the rapid
onset of unexplained
respiratory failure.

Background

Hantavirus
Pulmonary Syndrome (HPS) is a potentially deadly respiratory disease that
initially presents with flu-like symptoms. The cause is the Sin Nombre virus (SNV),
one of a number
Hantaviruses. It was first recognized during the outbreak in the Four Corners
Region in 1993. which caught the attention of health officials immediately
because it was associated
with a greater than 50 percent mortality . Hantaviruses are emerging pathogens
that have gained increasing attention over the past decade. These viruses are
members of the family Bunyaviridae and are grouped in a separate genus known as
Hantavirus. Serotypes Hantaan (HTN), Seoul (SEO), Puumala (PUU), and Dobrava
(DOB) virus cause hemorrhagic fever with renal syndrome (HFRS), a hemorrhagic
infection characterized by renal failure and shock. These Hantavirus
species are not known to cause disease in North America. The virus causing
HPS was identified as a novel Hantavirus, genetically distinct from those
previously identified. The virus was given several names including Little Water
virus, Four Corners virus, and Muerto Canyon virus before
Sin Nombre, Spanish for virus with no name, was selected.

Hantavirus is
carried by rodents. In the United States the deer mouse (Peromyscus
maniculatus)
and the cotton rat (Sigmodon hispidus) are the most common carriers. The animals shed
virus in their urine, feces, and saliva. Human infections result from inhaling
aerosolized urine, saliva, or powdered feces that contain the virus. Recent
investigations have found that many individuals who have become ill have had
repeated exposures to rodents or rodent droppings prior to infection. Less
common modes of infection are hand-to-mouth or hand-to-nose contact after
handling contaminated materials, or direct contact from a rodent bite. The
virus is not transmitted from person-to-person.

In the initial
outbreak, seventeen people suffered severe respiratory illness in the Four
Corners region of New Mexico, Arizona, Colorado and Utah. Since then clusters of
infections have arisen in both western and eastern parts of the United States,
as well as Canada. Other Hantaviruses causing HPS, such as the Bayou virus
and Black Creek Canal virus, have been isolated from individuals who had not
traveled to the Four Corners area. Geographic expansion of Hantavirus
reservoirs has been occurring over the past decade, leading to increased
morbidity and mortality from these viruses.

Recently
Hantavirus infections have occurred among wilderness travelers who come into
contact with rodent droppings. Although the illness continues to be relatively
rare, and mostly occurs
in rural areas of the western US, it continues to be associated with a high
mortality rate. The Centers for Disease Control cautions that “if you camp or
hike in an area inhabited
by rodents, you have a small risk of being exposed to infected rodents and
becoming infected with Hantavirus.”

Prevention

Individuals
who have an interaction with rodents or rodent droppings often are not aware of
it. An increased awareness of evidence of rodent activity can help prevent or
reduce contact with
potentially contaminated materials. Some “rules of the road” to minimize risk of
exposure include:

• Before settling into a wilderness cabin or enclosed shelter, it
should be opened, allowed to air, and inspected for rodents.

• If rodents appear to be nesting in a cabin or shelter, it should
not be used.

• Individuals camping in outdoor shelters or campsites should
examine the site for rodent droppings and burrows before making camp.

• Tents should not be pitched or sleeping bags placed in areas with
evidence of rodent feces or burrows.

• Tents and sleeping bags should be kept well away from potential
rodent shelters such as wood piles or garbage dumps.

• Tents should have floors or a tarp should be under sleeping bags
to avoid sleeping on bare ground.

• Food should be stored in rodent-proof containers .

• Waste should be removed from the camping area and discarded or
burned as appropriate.

• Water for drinking, cooking, cleaning dishes, and brushing teeth
should be disinfected by boiling, or by filtration and halogenation.

• Water should be kept in rodent-proof containers.

Hantavirus is
susceptible to chlorine bleach that can easily be included in the wilderness
traveler’s packing list. A diluted chlorine bleach solution can be used to wipe
down tables and counters before they are used for food preparation or dining.

Recognition

Initial
symptoms of HPS can appear as early as three days and as late as six weeks after
exposure. Most commonly symptoms begin within two weeks. Early presentation is a flu-like
syndrome that includes fever, headache, nausea and vomiting, as well as joint
and low back pain. During the 1993 outbreak the most common prodromal symptoms
were

fever and
myalgia (100 percent), cough or dyspnea (76 percent), gastrointestinal symptoms
(76 percent), and headache (71 percent). This early phase progresses for
approximately three to ten days.

As the
inflammatory response progresses in the lungs, fluid begins to accumulate and
causes noncardiogenic pulmonary edema. During this phase individuals develop dyspnea, hypoxemia, and tachycardia. Fulminant HPS is
characterized by bilateral interstitial pulmonary infiltrates and respiratory
compromise. Individuals in this stage usually require supplemental oxygen and
clinically resemble persons with acute respiratory distress syndrome (ARDS).
Individuals with advanced HPS can deteriorate quickly and generally are
hemodynamically unstable.

Early
recognition of signs of illness and management of potential exposures should be
emphasized. Since early symptoms are difficult to differentiate from other viral
illnesses, clues such as exposure to rodents or rodent droppings, geographic
location, or complaints of severe back and leg pain can help narrow the
differential. When exposure is suspected prior to the development of symptoms,
persons should inform their physician of the potential contact with contaminated
materials.

Management/Treatment

Pre-hospital
care of HPS consists of supportive measures including aggressive fluid
resuscitation, supplemental oxygen if available, and rapid transfer to a
tertiary care center with ICU facilities. In a wilderness situation evacuation
plans must be based on the distance from definitive care. The earlier initial
signs are recognized, the more time is available to transport the person to
medical attention before he becomes unstable.

Once in a more
stable setting, individuals with more advanced HPS require continued fluid
resuscitation and admission to the ICU for close observation and intervention.
Individuals with prodromal symptoms should be assessed, and admitted for
observation. No definitive treatment beyond supportive care is currently
available for this infection. The antiviral agent ribavirin may be
administered under a research protocol coordinated by CDC and the University of
New Mexico Health Sciences Center. This medication has been shown to be
beneficial for HFRS caused by other Hantaviruses, but its role in HPS is still under investigation.

Prognosis

With increased
recognition of the disease and more aggressive interventions for hemodynamic instability, the mortality rate for HPS has decreased
to approximately 38 percent. Persons surviving the fulminant phase of the
disease generally do very well and often recover with little or no residual
deficits. Several months of fatigue and decreased exercise tolerance seem to be
the most common sequelae of HPS.